As renewable energy has become an important economic and environmental imperative, the use of wind turbines to generate electricity on a commercial scale has assumed a role of increasing importance in the national and international arenas. While wind turbines, and the aggregation of wind turbines known as “wind farms,” have been in existence for years, current efforts to increase electrical generation capacity using existing equipment is a major focus for development in power systems. Although environmental conditions, such as lack of wind, gusting winds, sand storms, icing conditions, and extreme heat, are factors that cannot be controlled to any significant extent, there are modifications that can be made to existing systems that will improve the overall electricity generation capacity of such systems. Amongst these modifications are active wind flow modification techniques and devices, including the use of wind turbine blade load control technologies such as plasma actuators, and advanced pitch control. These technologies require sophisticated controls and equipment, consume energy, are subject to failure of components, and are more costly than static methods. A simpler and more reliable solution is the retrofitting and use of static devices such as vortex generators and winglets to increase the efficiency of wind turbine blades during normal operation. A properly designed vortex generator system optimizes the design and spacing of the vortex generators to allow a more complete mixing of airflow along currently underutilized blade sections.
Vortex generators (“VGs”) have a long history of use, primarily in the aircraft industry. They are employed to increase the efficiency of an airfoil by creating vortices on the low pressure side of an airfoil that will introduce energy into the boundary layer adjacent the surface, thereby moving downstream the point at which the boundary layer will break away from the surface and the airfoil will cease to provide lift, i.e., the point at which stall occurs. For a given set of aerodynamic and environmental conditions, the addition of VGs to an otherwise smooth wing surface can result in a significant increase in the efficiency of the wing.
VGs can also be employed to beneficial effect on wind turbine blades, although the parameters affecting such use must be modified to compensate for differences between airflow over an airplane wing and airflow over a rotating wind turbine blade. Such differences relate primarily to the fact that a wind turbine blade, which rotates about a hub to which the blade root is connected, experiences relatively high wind velocities toward the blade tip and low wind velocities nearer the root, whereas an aircraft wing experiences an essentially constant airspeed along its length. As a result, typical parameters and “rules of thumb” that help to determine the placement and configuration of vortex generators on an airplane wing are not adequate or sufficiently reliable for determining the efficient placement and configuration of vortex generators on a turbine blade. The optimal design and placement of VGs on a turbine blade may vary along the length of the blade, depending upon blade configuration, anticipated wind conditions, and turbine design requirements. In general, the greater the relative wind velocity near the tip of a turbine blade, the less likely it is that that section of the blade will encounter stall conditions. Conversely, analyzing wind conditions closer to the root, the velocity of the relative wind decreases closer to the hub while, simultaneously, structural considerations require a thickening of the blade. The result is that the blade may become an inefficient airfoil near the root, and incipient or actual stall conditions may significantly diminish the efficiency of the inner sections of the blade. When these and other factors are taken into consideration, properly designed and positioned VGs can significantly increase the electrical output of a wind turbine generator for a given ambient wind speed, particularly within a blade section that is near to the blade root.
In order to increase wind turbine efficiency through the use of VGs, ideally, one would like wind turbine blade design information, including the airflow distribution along the blade. Such design details, however, are generally not disclosed by wind turbine blade manufacturers and may not be available upon request. In order to properly simulate the aerodynamic effects of the VGs on a given blade of unknown characteristics, it is first desirable to identify certain blade parameters such as the blade chord length, blade airfoil curvature, and wind flow patterns over the blade surface. In the design and testing of a new blade, much of this information may be obtained, or supplemented, through computer and wind tunnel simulations. Where such simulations are not a practical solution, however, such as where a system is already in existence and is currently operating in the field, a different methodology may be employed.
U.S. Pat. No. 6,065,334 to Corten discloses a method of visualizing fluid flow across a surface that has one or more flap-like members, and a hinge connecting the flap to the surface. The invention is intended to provide a visual indication of the area on an aerodynamic surface at which wind flowing over the surface will separate from the surface under various conditions airflow speed and relative wind angle. This point will provide information about the point at which a stall condition occurs. In Corten, the point of separation is indicated, when the turbine blade is operating, by the flap-like members being caused to rotate on their hinges by a backwards airflow behind the separation point. Where the flap-like members are given a different color on each side, the visual representation of the point of separation will be more easily discerned.
Although Corten's method provides a visual representation showing the point of airflow separation on an aerodynamic surface, the method requires cumbersome preparation and installation of the flap-like members, and provides only visual information showing the point of separation, but does not necessarily indicate where vortex generators should be located for optimum effectiveness. The mere cumbersomeness of Corten's method makes it impractical for use on existing wind turbines having blades whose aerodynamic characteristics are unknown.
A less elaborate but equally effective method of obtaining information regarding a given installed wind turbine unit is to use tell-tales (short pieces of yarn or string attached at one end to the surface of a turbine blade and freely detached at the other end) to indicate the direction of any relative wind. Tell-tales are old in the art, having been used to indicate airflow separation on aircraft wings for decades.
A significant problem facing the wind power generation industry is that necessary information regarding the aerodynamics of existing installations may be lacking. While it is theoretically possible to run wind tunnel tests and computer simulations to determine the critical aerodynamic characteristics of a given airfoil or blade, the large size and permanent emplacement of existing systems present significant obstacles to the use of wind tunnel testing and computer simulations to design retrofit enhancements for such systems. What is needed is a methodology that is suitable for use on existing, permanent installations, and that provides useful information regarding vortex generator placement whereby the power-generation capacity of such systems may be increased without the need for costly and complex equipment and analyses.